//没有return的默认返回undefined
function add() {
  console.log("hello");
}

let a = add();
console.log(a); //undefined

//return为空的隐式返回undefined
function noReturn() {
  return;
}
let b = noReturn();
console.log(b); //undefined

//访问对象不存在的属性，得到undefined
let obj = {};
console.log(obj.hello);

//变量声明未赋值，也为空
let c;
console.log(c);
